As a device capable of recording large capacity data, a magnetic recording apparatus is a basic technology to support the present high information society. In the magnetic recording apparatus, data on a recording medium is read and written by a magnetic head. In order to increase the recording capacity per unit area of a magnetic disk, it is necessary to raise bit areal recording density. In order to increase the recording density, it is necessary to increase linear recording density and track density while a specified signal-to-noise ratio is ensured.
In order to improve the linear recording density, it is effective to reduce a media noise caused by irregularity of a recording pattern recorded on the medium and to improve signal quality by improving the readback signal resolution. The magnetic recording medium is an aggregate of magnetic crystal grains. Although the media noise can be reduced by micronizing the magnetic crystal grains, a problem occurs in that the grains become thermally unstable. Accordingly, in order to micronize the crystal grains while the thermal stability is ensured, it is effective to increase magnetic anisotropic energy Ku of the medium. However, the increase of the magnetic anisotropic energy Ku for inversion to increase causes a switching magnetic field, and accordingly, the recording magnetic field intensity of a head is also increased.
On the other hand, one way to increase the track density is to narrow the recording core width of the magnetic recording head. However, the narrowing of the recording core width reduces the recording magnetic field intensity. Besides, there is a limit in increase of the recording magnetic field intensity by the optimization of the recording magnetic pole material and the structure of the recording head. When the recording magnetic field intensity is insufficient as compared with the switching magnetic field of the medium, that is, when the recording capacity is lowered, part of bits cannot be inverted, and the signal quality is reduced.
Thus, it is difficult to micronize the crystal grains of the recording medium and to narrow the track width of the recording head while the thermal stability of the recording medium and the recording capacity of the recording head are maintained. This is because the improvement of the signal quality, the securing of the thermal stability of the recording medium, and the maintaining of the recording capacity of the recording head trade-off. This problem is called a trilemma. As a method of resolving the trilemma, the following two methods are considered.
As the first method of solving the trilemma, a heat assist magnetic recording system is proposed in which a recording medium having a large magnetic anisotropic energy Ku is used, and at the same time as the timing when a recording magnetic field is applied or just before that, the medium is heated to temporarily reduce the switching magnetic field and recording is performed. See Japan Journal of Applied Physics, 38, Part 1, 1839 (1999). In the heat assist magnetic recording apparatus, it is desirable that the spot size of irradiated light is a size comparable to the recording width of a recording pattern. This is because, when the spot size of light is excessively larger than the recording width, a track adjacent to the recorded track is also heated, and the thermal stability is degraded. Since the bit size of the magnetic recording apparatus is smaller than the wavelength of light, a minute area not larger than the wavelength is heated. As a technique to heat such a minute area, a technique of generating a near-field light is proposed. The near-field light is a localized electromagnetic field existing in the vicinity of a minute material not larger than the light wavelength, and is generated by using a minute, aperture having a diameter not larger than the light wavelength or a light scattering body of metal. See Jap. Pat. Appl. No. JP-A-2007-128573. Incidentally, the spot size is defined as a width in which the amount of reduction of the switching magnetic field of a magnetic recording medium is half the amount of reduction at the heating center position.
As the second method, there is a shingle recording system. See U.S. Pat. No. 6,185,063. This is different from a related art random access system, and is a method in which overwriting is performed in one direction of a track width direction from a certain track to an adjacent track. Even if a recording track width is wide, a next track is overwritten while a part of a track on one side remains, and therefore, a track narrower than the recording track width can be formed. Thus, the core width is not always narrowed, and the problem where the writing capacity of the head is reduced can be avoided.
The readback signal resolution in the magnetic recording apparatus is determined by the transition width between bits of a recording pattern and the resolution of readback sensitivity of the reproducing head. In order to raise the resolution of the recording pattern, irrespective of a recording system, a rectangular shape of the recording pattern is preferred. This is because when a recording pattern which does not have a recording system, there is a problem that the transition shape of the recording pattern recorded on the recording medium by the rectangular shape is reproduced by a giant magnetoresistive (GMR) head or a tunneling magnetoresistive (TMR) head, the transition width between bits appear large, and the readback signal resolution is reduced. However, in current recording systems, there is a problem that the transition shape of the recording pattern recorded on the recording medium by the recording head is curved at a track edge as compared with the track center.
This is due to the recording magnetic field distribution from the recording head not being rectangular. The transition position of the recording bit is determined by a line where the switching magnetic field of the medium at a downstream side in a medium rotation direction, i.e., at a trailing side, is equal to the recording magnetic field. As shown in FIG. 1A, an example having a contour 21 of a recording magnetic field intensity generated from a magnetic pole has a distribution which is expanded toward the outside. Thus, at the track edge of the recording pattern, the phase of the transition position advances as compared with the track center, and as shown in FIG. 1B, the transition shape of the recording pattern 31 has a curved shape. The transition curvature of the recording pattern as stated above is clarified by observation using a magnetic force microscope. See Japan Journal of Applied Journal of Magnetism and Magnetic Materials. Vol. 303, pp. 271-275 (2006). That the transition shape of the recording patterns is curved as stated above is called magnetization transition curvature, and a distance 34 between a position where the phase is most delayed and a position where the phase is most advanced is called a magnetization transition curvature amount. An arrow 32 indicates a head traveling direction, and an arrow 35 indicates a tract width direction.